Add tests for Workflow objects

[nvaytet]

Following #136, the main way to perform a reduction is to use the Workflow objects (e.g. Sans2dTutorialWorkflow).
We have kept the old unit tests that work on the Pipeline and providers, but we should also test the workflow wrappers.

This is probably not much more than ensuring all the default parameters are set up correctly?

[SimonHeybrock]

Recent refactoring kind of did this implicitly --- most test now create pipelines using the pre-defined workflows.